I would dress her in the spare collar and leash that I always carry (though I finally got the emergency muzzle and booties out of my pack after Dugan four-legs was sidelined), then hike out with her.
I would leave a note with thorough description, if there is a signboard at the trailhead to do so.
If the hours were such that I could notify the local authorities, I would do so. Otherwise, I would bring her home, keeping her isolated from my pets (better safe than sorry until a health screening occurs), then call the local police during their next business hours.
If she hadn't been reported, my next step would be my local shelter or vet, to scan for microchip. I would check the dog for a tattoo (lip, ear, or inside of rear leg are the most common places).
If still nothing, I would make a flier with a color picture and full description, then post them at the trailhead and closest towns where I found her. I would also notify all the local vets, shelters, and hiking organizations in the area.
I would do this even if the dog had a collar and tags. One piece of advice with tags - MAKE SURE they are ALWAYS up to date. Dugan doesn't hike without: rabies tag, town license, id tag with his name and my name/address/phone, microchip notice, tattoo notice. If you let your dog hike off leash, it isn't a bad idea to add a small cowbell. If you can find one of those little pouches that attaches to a collar, it could be useful to keep a small note with emergency contacts, health information, and such. And as always - carry a copy of the rabies certificate. A rabies tag or town license is NOT proof of rabies vaccination in all states.
If this sounds familiar, it should, considering the recent lost dog thread. This is looking for a lost dog... in reverse.